Toolbars with the TBSTYLE_FLAT style are essentially transparent so will possess the same 'background' colour as whatever is underneath. So, if the parent of the toolbar, or some other window immediately below the toolbar, is white, you'll need to change the background colour beneath the toolbar to whatever colour you want (if you want default behaviour then use GetSysColor(COLOR_BTNFACE) for that colour). Alternatively, just lose the TBSTYLE_FLAT toolbar style.